Simulate synthetic events using ReactTestUtils

[sophiebits]

Most of the time this is what you want to do, so I've renamed what was Simulate to be SimulateNative.

Now Simulate.change does what you expect, so this fixes #517 and fixes #519.

Includes/depends on #937.

[sophiebits]

/cc @chenglou @yungsters. Looks like we should probably be recreating ReactTestUtils.Simulate whenever a new event plugin is injected. Ideas for a clean API for doing so?

[zpao]

I like. Suggestions for changing internally? I'm guessing we could just codemod existing callsites to be SimulateNative.* and move on, though we may want to do a pass and see if some of those should be testing the synthetic.

[sophiebits]

My guess is few or none of the existing callsites should be using SimulateNative; simulating native events is useful when testing React itself or event plugins but otherwise I don't think it should be needed. If you send me more info about any failing tests, that would be helpful -- it seems likely that there are changes that could be made to this code.